Method for surface mounting electrical components to a substrate

ABSTRACT

A method for surface mounting electrical components to a substrate, such as a printed circuitboard, involves use of an anisotropically conductive adhesive or Z-Axis adhesive between facing conductive surface areas on the component and substrate. Pressure is applied to the conductive adhesive by a nonconducting adhesive that is first cured between oppositely facing nonconductive surface areas of the component and substrate. This fixes the thickness of each layer of the conductive adhesive at a dimension no greater than its design conductive thickness. In a first submethod, the nonconducting adhesive is a fast setting adhesive subjected to mechanical pressure only as it is assembled on the substrate prior to the subsequent curing of the conductive adhesive. In a second submethod, it is a high shrinkage adhesive that applies compressive force between the component and substrate as it cures and shrinks dimensionally while at a temperature below the subsequent curing temperature of the conductive adhesive.

RELATED PATENT DATA

This patent resulted from a file wrapper continuation application ofU.S. application Ser. No. 08/538,826, filed on Oct. 5, 1995, entitled"Method For Surface Mounting Electrical Components to a Substrate"listing the inventor as Mark E. Tuttle, and now abandoned, which is acontinuation application of U.S. application Ser. No. 08/147,495, filedon Nov. 5, 1993, entitled "Method for Surface Mounting ElectricalComponents to a Substrate" listing the inventor as Mark E. Tuttle, andnow abandoned.

TECHNICAL FIELD

This disclosure pertains to methods for surface mounting electricalcomponents to a substrate by use of directionally conductive adhesives.

BACKGROUND OF THE INVENTION

Adhesive surface mounting of devices to a supporting substrate isexemplified by the disclosure of U.S. Pat. No. 3,818,279 to Seeger, Jr.et al. It discloses use of a conductive elastomeric material containingelectrically conductive particles for establishing random conductivepaths between a substrate area and an overlying device. Such adhesiveshave no directionally conductive properties.

U.S. Pat. No. 4,339,785 discloses the mounting of a component to aprinted circuit board by use of a structural adhesive.

U.S. Pat. No. 4,774,634 to Tate et al., discloses use of a structuraladhesive connecting to the body of a device and used in combination witha flexible conductive adhesive at electrical contact areas. Thiscombination produces a conductive connection between the leads of acircuit component and a supporting substrate, but the connection isagain multidirectional.

The need to confine the area of conductivity when surface mounting acomponent by use of a conductive adhesive has led to development ofanisotropically conductive adhesives, also known as Z-axis adhesives.Anisotropically conductive adhesives are a mixture of a nonconductiveadhesive binder and conductive particles capable of forming electricallyconductive paths between facing conductive surface areas when subjectedto heat and pressure. By delineating the opposed areas at which pressureis exerted on such an adhesive, one can eliminate stray and unwantedconductive paths outside that area, where the conductive particles willbe sufficiently separated from each other so that current will not flowthrough the composite mass. This eliminates the flow of current betweenadjacent conductive areas on the same substrate and between facingconductive surfaces separated by a distance greater than the minimaldistance required to complete an electrical path through the adhesivemixture.

Surface mounting of components by use of an anisotropically conductiveadhesive currently requires application of pressure during the course ofsetting or curing the adhesive. This typically requires usage ofmechanical fixtures that must remain in place as the component andsubstrate are heated within an oven. Present use of such adhesives alsorequires redesign of the mechanical fixtures as any component featureswithin a circuit are changed by subsequent developments.

The present invention has been developed to take advantage of theability to apply conductive adhesive by screen printing or other typesof printing technology. The use of printing technology to createelectrical connections between components and a substrate is very rapidand economical. The inventive method also provides a high degree offlexibility for meeting changing circuit requirements, eliminating thenecessity of redesigning mechanical fixtures to hold the circuitcomponents in place as the conductive adhesive is being set.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the accompanying drawings, which are briefly describedbelow.

The drawing is a schematic cross-sectional view illustrating themounting of electrical component on a supporting substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws "to promote the progressof science and useful arts" (Article 1, Section 8).

The present method is schematically illustrated by the accompanyingdrawing. It can be utilized for surface mounting an overlying electricalcomponent 10 to a supporting substrate 11. The component 10 andsubstrate 11 have oppositely facing surfaces that include correspondingconductive surface areas 12, 13 and 14, 15, respectively. The pairedcorresponding conductive surface areas establish electrically conductivepaths between the component 10 and substrate 11.

A layer of anisotropically conductive adhesive 16 is applied to at leastone surface in each of the paired conductive surface areas 12, 13 and14, 15. Such an adhesive includes a mixture of a nonconductive adhesivebinder and electrically conductive particles. The conductive particleswithin the adhesive are capable of forming electrically conductive pathsbetween the surface areas 12, 13 and 14, 15 when subjected to heat andpressure to reduce the layer thickness to an operable conductivedimension. This thickness dimension is a function of the conductiveparticle sizes within the composite adhesive mass.

Anisotropically conductive adhesives, also known as Z-axis adhesives,are well known in surface mounting technology for electronic components.A more comprehensive description of such compositions can be foundwithin the disclosure of U.S. Pat. No. 4,588,456, which was issued May13, 1986 and which is hereby incorporated into this disclosure byreference.

One specific example of an anisotropically conductive adhesive amenableto the practice of this new method is Zymet ZXUV 101, a Z-Axis epoxyadhesive manufactured by Zymet, Inc., of East Hanover, N.J. Otherexamples are ZME series Z-Axis epoxy adhesive paste and ZSP seriesZ-Axis epoxy adhesive paste, both manufactured by Al Technology, Inc. ofLawrenceville, N.J.

In place of the conventional step of mechanically applying pressure tothe anisotropically conductive adhesive layers 16 by using mechanicalfixtures or devices that must remain in place to exert pressure on theassembly as the adhesive is set by curing within an oven, the presentinvention utilizes a second adhesive to assure application of adequatecuring pressure for the conductive adhesive. This second adhesive is anonconductive adhesive 17 applied to at least one of the oppositelyfacing surfaces of the component 10 and substrate 11 at a locationspaced from the conductive surface areas 12, 13 and 14, 15.

In the case of a symmetrical circuit component 10, adhesive 17 might beapplied at its center, assuming that no conductive surfaces are at thislocation. Adhesive 17 might also be applied about multiple locations ona particular electrical component 10, depending upon its size andsurface configuration.

The choice of adhesive 17 will be dependent upon the specific manner bywhich pressure is to be applied to the adhesive layers 16 during thetime of curing.

The respective thicknesses of the adhesive layers should be such thatthe thickness of the applied anisotropically conductive adhesive layers16 is only slightly greater than the operable conductive dimensions atwhich conductive paths are formed across their composite masses. Afterapplication of adhesive layers 16 and 17, the component 10 and substrate11 are next assembled in their desired overlying positions relative toone another. The corresponding conductive surface areas 12, 13 and 14,15 will then be in spatial registration with one another. At this pointthe anisotropically conductive adhesive will be in surface-to-surfacecontact with the corresponding conductive surface areas 12, 13 and 14,15 and the nonconductive adhesive will be in surface-to-surface contactwith oppositely facing nonconductive surfaces of component 10 and 11.Illustrative nonconductive surfaces are designated in the drawing by thereference numerals 18 and 19, respectively.

Curing of the adhesive layers 16 and 17 is a sequential two-stepprocess. First, the nonconductive adhesive layer 17 is cured to bond thecomponent 10 to the substrate 11 in a manner that subjects theanisotropically conductive adhesive to pressure as a result of thebonding forces applied to it by the set nonconductive adhesive. Theassembling equipment for placing and/or holding component 10 on thesubstrate 11 can then be released. The anisotropically conductiveadhesive is subsequently cured without further external curing pressurebeing applied to component 10. This results in bonding of thecorresponding conductive surface areas 12, 13 and 14, 15 to one another,causing the conductive particles within the anisotropically conductiveadhesive to form electrically conductive paths between the correspondingconductive surface areas as a result of the holding pressure of adhesive17 while adhesive layers 16 are being cured. The pressure applied to thelayers of adhesive 16 by action of the cured adhesive 17 reduces thethickness of the conductive adhesive to the operable conductivedimension at which conductive paths are completed through the particleswithin the adhesive composite during its curing process.

Two submethods for effectively setting the anisotropically conductiveadhesive 16 under the curing pressure imparted by action of adhesive 17have been identified to this point.

The first submethod involves use of a high shrinkage nonconductiveadhesive that cures at an energy level or temperature substantiallylower than that required to set or cure the anisotropically conductiveadhesive. Conversely, the anisotropically conductive adhesive is curableat an energy level or temperature substantially greater than thatrequired to cure the high shrinkage nonconductive adhesive. Thissubmethod requires the component 10 to be placed on substrate 11 at aposition wherein the thickness of adhesive layers 16 is not greater thanthe sum of its design conductive thickness plus expected shrinkageduring the setting of adhesive 17. The shrinkage that occurs in the highshrinkage nonconductive adhesive will result in the exertion ofcompressive forces between the component 10 and substrate 11. Theseforces will pull the two elements toward one another and apply therequired compression to the uncured anisotropically conductive adhesivelayers 16 to reduce their thicknesses to the dimensions at which theyform conductive paths. After the nonconductive adhesive 17 has cured andshrunk, these compressive forces will remain intact as the assembly isfurther heated or subjected to radiation or other energy to set or curethe anisotropically conductive adhesive layers 16. Curing of adhesivelayers 16 can be accomplished in the absence of external forces sincethe adhesive layers 16 will continue to be subjected to the pressureexerted by the previously-cured adhesive layer 17.

The second submethod involves use of a very fast setting nonconductiveadhesive 17 to quickly bond the component 10 to the substrate 11 duringthe component placement step. The required curing pressure to achieveelectrical contact through the particles within the anisotropicallyconductive layers 16 will be provided externally to the component 10through conventional component placement equipment not shown thatspatially locates the component 10 relative to substrate 11 at thedesign conductive thickness of adhesive layers 16. After the fastsetting adhesive 17 has cured, this pressure will be maintained duringsubsequent curing of the anisotropically conductive adhesive layers 16without any requirement of continuing external forces being applied tocomponent 10.

A typical Z-axis epoxy adhesive, as identified previously, will cure atelevated temperatures of approximately 150° C. The identified Zymetadhesive requires a pressure of 0.5-1.0 Kgm/cm² during curing to obtaingood electrical contact. The AI Technology adhesives specify applicationof 0.30 Kgm/cm² during curing at 150°-160° C.

Such pressures can be achieved by use of the first-described submethodwhen utilizing a nonconductive adhesive 17 that shrinks approximately4-12 percent as it is curing and that will set or cure at a temperaturebelow that necessary to set the anisotropically conductive adhesivelayers 16. Adhesives that shrink approximately 4-12 percent as they arecured will exert adequate pressure to set and activate availableanisotropically conductive adhesives in adjacent areas about rigidelectronic components and substrates, such as a printed circuitboard.Specific examples of such high shrinkage nonconductive adhesives thatcan be used in this process are Loctite 420 adhesive, manufactured bythe Loctite Corporation, of Newington, Conn. and Pacer M5 adhesive,manufactured by Pacer Technology, of Rancho Cucamonga, Calif. Loctite420 is a cyanoacrylate adhesive having 10 percent shrinkage duringcuring, and cures at elevated temperatures below 150° C. Pacer M5 hassimilar properties.

Examples of fast curing adhesives amenable to the second submethod areLoctite UV curable acrylics, produced by Loctite Corporation ofNewington, Conn. and Emcast 1720 epoxy adhesives, manufactured byElectronic Materials, Inc. of New Milford, Conn. Both are cured in lessthan 10 seconds by application of ultraviolet radiation. They can beflashed with ultraviolet radiation while the component 10 is being heldmechanically under pressure against the substrate 11 as the component 10is placed on the substrate 11 and with the thicknesses of the layers ofadhesive 16 at the operable conductive dimensions at which conductivepaths are completed through the particles within the adhesive composite.The assembly can then be transferred to an oven to subsequently cure theanisotropically conductive adhesive layers 16 in the absence of furtherexternal pressure. The required pressure will be maintained on thelayers of adhesive 16 as they are being cured by operation of thepreviously-cured second adhesive layer 17.

The nonconductive adhesive 17 will normally constitute a thermosettingadhesive that can be cured below the curing temperature of theanisotropically conductive adhesive layers 16. The nature and amount ofenergy utilized for setting the nonconductive adhesive will be dependentupon the specific choice of adhesive. Numerous adhesives arecommercially available that meet the physical requirements of bothdescribed submethods.

The nonconductive adhesive might be cured by application of anycompatible form of energy, including ultraviolet radiation or convectiveheat. Similarly, the anisotropically conductive adhesive, which istypically comprised of a thermosetting binder, can be heated by anydesired radiation or heat source, such as an oven. The details of curingboth types of adhesives are well known and need not be further developedherein in order to enable those skilled in this field to practice theinvention as described.

In compliance with the statute, the invention has been described inlanguage more or less specific as to methodical features. It is to beunderstood, however, that the invention is not limited to the specificfeatures described, since the means herein disclosed comprise preferredforms of putting the invention into effect. The invention is, therefore,claimed in any of its forms or modifications within the proper scope ofthe appended claims appropriately interpreted in accordance with thedoctrine of equivalents.

I claim:
 1. A method for mounting an electrical component to asupporting substrate comprising the following steps:providing anelectrical component, the electrical component having a nonconductivesurface and a pair of conductive surfaces adjacent the nonconductivesurface, the pair of conductive surfaces of the electrical componentbeing a first conductive surface and a second conductive surface;providing a substrate, the substrate having a nonconductive surface anda pair of conductive surfaces adjacent the nonconductive surface, thepair of conductive surfaces of the substrate being a third conductivesurface and a fourth conductive surface; applying a nonconductiveadhesive in contact with the nonconductive surface of the substrate andin contact with the nonconductive surface of the electrical component,the nonconductive adhesive not being in contact with the first, second,third or fourth conductive surfaces, the nonconductive adhesive curingat a first temperature, the nonconductive adhesive shrinking as itcures; applying an anisotropically conductive adhesive in contact withthe first conductive surface and in contact with the third conductivesurface, the anisotropically conductive adhesive curing at a secondtemperature, the second temperature being higher than the firsttemperature; applying the anisotropically conductive adhesive in contactwith the second conductive surface and in contact with the fourthconductive surface; curing the nonconductive adhesive at the firsttemperature to bond the nonconductive surface of the substrate to thenonconductive surface of the electrical component, the nonconductiveadhesive shrinking as it cures, the anisotropically conductive adhesivebeing compressed between the first and third conductive surfaces andbetween the second and fourth conductive surfaces as the nonconductiveadhesive shrinks; and after curing the nonconductive adhesive, curingthe compressed anisotropically conductive adhesive at the secondtemperature to 1) form an electrically conductive path between the firstand third conductive surfaces and bond the first conductive surface tothe third conductive surface and 2) form an electrically conductive pathbetween the second and fourth conductive surfaces and bond the secondconductive surface to the fourth conductive surface.
 2. A method formounting an electrical component to a supporting substrate comprisingthe following steps:providing an electrical component, the electricalcomponent having a nonconductive surface and a pair of conductivesurfaces adjacent the nonconductive surface, the pair of conductivesurfaces of the electrical component being a first conductive surfaceand a second conductive surface; providing a substrate, the substratehaving a nonconductive surface and a pair of conductive surfacesadjacent the nonconductive surface, the pair of conductive surfaces ofthe substrate being a third conductive surface and a fourth conductivesurface; applying a nonconductive adhesive in contact with thenonconductive surface of the substrate and in contact with thenonconductive surface of the electrical component, the nonconductiveadhesive curing under first conditions; applying an anisotropicallyconductive adhesive in contact with the first conductive surface and incontact with the third conductive surface, the anisotropicallyconductive adhesive curing under second conditions, but not curing underthe first conditions; applying the anisotropically conductive adhesivein contact with the second conductive surface and in contact with thefourth conductive surface; curing the nonconductive adhesive under thefirst conditions to bond the nonconductive surface of the substrate tothe nonconductive surface of the electrical component, the nonconductiveadhesive being compressed between the nonconductive surfaces of theelectrical component and the substrate by external mechanical forcesduring the curing of the nonconductive adhesive, the anisotropicallyconductive adhesive being compressed between the first and thirdconductive surfaces and between the second and fourth conductivesurfaces by external mechanical forces as the nonconductive adhesivecures; after curing the nonconductive adhesive, removing the compressingof the external mechanical forces; and after curing the nonconductiveadhesive and removing the compressing of the external mechanical forces,and while not providing external mechanical forces to further compressthe anisotropically conductive adhesive, curing the compressedanisotropically conductive adhesive under the second conditions to 1)form an electrically conductive path between the first and thirdconductive surfaces and bond the first conductive surface to the thirdconductive surface and 2) form an electrically conductive path betweenthe second and fourth conductive surfaces and bond the second conductivesurface to the fourth conductive surface.
 3. The method of claim 2wherein the first conditions comprise exposure to ultraviolet light.